This invention relates to the treatment of combustion exhaust gases, more particularly, to a method and apparatus for the treatment of combustion exhaust gases, whereby dioxins and nitrogen oxides in combustion exhaust gases from waste burning boilers and combustion furnaces are treated with a catalyst at comparatively low temperatures.
A system for the conventional treatment of combustion exhaust gases from electricity generation with waste is shown schematically in FIG. 3 and a system for the treatment of combustion exhaust gases from commercial and captive thermal power generation is shown schematically in FIG. 4.
As shown in FIG. 3, the treatment of combustion exhaust gases from electricity generation with waste is chiefly intended to remove dust, heavy metals, SOx, HCl, etc. in the exhaust gases and to this end a dry dust collector (electrostatic precipitator or bag filter) 3 is connected to a downstream wet dust collector (scrubber) 10, where the combustion exhaust gas is cleaned and its temperature is lowered; thereafter, the combustion exhaust gas has its temperature elevated in an exhaust gas heater (steam heater) 11 before it is fed into a de-NOx and dioxin decomposing (catalyst) unit 12.
This prior art technology involves following problems:
(1) The number of equipments is large and the system operation becomes complicated; PA1 (2) The construction and running costs of the facilities are high; PA1 (3) Much labor is required for system maintenance and management services; PA1 (4) Reheating the combustion exhaust gas after cooling in the wet dust collector reduces thermal efficiency (or plant thermal efficiency in power generation); and PA1 (5) The efficiency of dioxin and NOx removal is low. PA1 (1) The temperature of the combustion exhaust gas is as high as 300-350.degree. C., so there is a potential for the resynthesis of dioxins in a cooling process (within the air preheater 2 or other apparatus at a subsequent stage that involves the process of transition from high temperature to less than 300.degree. C.); and PA1 (2) The de-NOx unit 12 which is installed upstream of the dust collector 3 is liable to suffer from catalyst poisoning and contamination with dust if it is applied to waste burning boilers.
In the conventional treatment of combustion exhaust gases from commercial and captive thermal power generation, the fuel characteristics are constant and, therefore, as shown in FIG. 4, the de-NOx (the term "de-NOx" means "selective catalytic reduction to reduce NOx in flue gas to N.sub.2) unit 12 is installed in the hot zone at the exit of the boiler 1 to achieve high de-NOx efficiency. However, even this technology involves the following problems:
Catalytic dioxin decomposition has not heretofore been commercialized since it is low in economy, efficiency and low-temperature activity.
On the other hand, it has been known that the de-NOx catalyst is capable of decomposing dioxins although the efficiency is very low. In order to achieve high-efficiency removal of dioxins, a "two-stage" system has been adopted; as shown in FIG. 5, this system is characterized by adding slaked lime or activated carbon 7 to the combustion exhaust gas which is then passed through a bag filter 3 and a coke filter 15 successively.
FIG. 6 shows the result of dioxin removal using slaked lime or activated carbon in accordance with the scheme shown in FIG. 5. According to FIG. 6, an exhaust gas having a dioxin concentration of 10 ng-TEQ/Nm.sup.3 (TEQ: 2,3,7,8-tetrachlorodibenzodioxin) can be treated so as to lower the dioxin concentration to 0.1 ng-TEQ/Nm.sup.3. The dioxin concentration to 0.5 ng-TEQ/Nm.sup.3 is the guideline value of Ministry of Health and Welfare (MHW) in Japan. Considering the characteristics of dioxins and activated carbon, the temperature of the exhaust gas should desirably be kept to 150.degree. C. or below. In addition, the two-stage process not only involves complicated system construction but also lacks rationality from the viewpoint of energy recovery.